The present invention relates to an electrolyte for secondary battery which is a polymer electrolyte and a secondary battery comprising such an electrolyte for secondary battery and more particularly to an electrolyte for secondary battery which is a polymer electrolyte useful as a material for electrochemical device such as battery showing a high ionic conductivity and a secondary battery comprising such an electrolyte.
In recent years, there has been a growing demand for the enhancement of performance of and the reduction of size of electronic products. Thus, the battery material as an energy source for these electronic products have been required to have a smaller size, a lighter weight and a higher capacity and energy density. Various studies and developments have been underway.
In recent years, as such an energy source for electronic products there has been used a lithium secondary battery. A lithium secondary battery normally comprises a metal oxide as a cathode, a carbon-based material or the like as an anode, and a separator and an electrolyte interposed therebetween. This is a secondary battery having a high energy density. However, since the lithium secondary battery comprises an electrolyte, it has a safety problem arising from electrolyte leakage. Further, since the lithium secondary battery needs to comprise a metal can as an outer case to prevent electrolyte leakage, it finds difficulty in reducing the weight thereof.
In order to overcome shortcomings caused by the use of electrolyte, studies have been made of so-called polymer electrolyte, i.e., electrolyte comprising a polymer compound. A polymer electrolyte is flexible and thus can follow mechanical impact and even the change of volume of electrode caused by ion-electron exchange reaction between electrode and electrolyte.
As such a polymer electrolyte there has been proposed in U.S. Pat. No. 4,303,748 a solid electrolyte having an alkaline metal salt or alkaline earth metal salt dissolved in a polyalkylene oxide. However, this solid electrolyte has an insufficient ionic conductivity and a high contact resistance with respect to electrode material and thus leaves something to be desired. Such an electrolyte having an insufficient ionic conductivity cannot provide a sufficient current density during charge and discharge and thus cannot be put to use in purposes requiring a large current, restricting the use.
On the other hand, a system called gel electrolyte having an organic polymer compound impregnated with a non-aqueous organic solvent has been proposed in Japanese Patent Publication No. 1986-23945, Japanese Patent Publication No. 1986-23947, and U.S. Pat. Nos. 4,830,939 and 5,429,891. Such a gel electrolyte comprises an electrolytic solution encapsulated in a polymer compound as a matrix to have an ionic conductivity. However, a problem arises that the retention of electrolytic solution can be lost or deteriorated with the change of temperature. Further, in order to obtain a high ionic conductivity, it is necessary that the polymer compound be impregnated with a larger amount of electrolytic solution. In this case, however, it is likely that the retention of electrolytic solution can be deteriorated or the electrolyte leakage can occur.
Moreover, J. Polymer Science, xe2x80x9cPolymer Physicsxe2x80x9d, vol. 21, page 939, 1983 proposes a polymer electrolyte comprising a polyacrylonitrile as a substrate. When impregnated with a non-aqueous organic solvent, this polymer electrolyte exhibits a certain conductivity. However, when not impregnated with a non-aqueous organic solvent, this polymer electrolyte exhibits insufficient ionic conductivity. Thus, similarly to the aforementioned polymer electrolyte, this polymer electrolyte cannot provides sufficient current density during charge and discharge and cannot be put to use in purposes requiring a large current, restricting the use.
An aim of the invention is to provide an electrolyte for secondary battery which is a polymer electrolyte having a high ionic conductivity and an excellent safety useful as a material for electrochemical device such as secondary battery and a secondary battery having an excellent cycle life performance comprising such an electrolyte.
In other words, the invention lies in:
(a) An electrolyte for secondary battery comprising an ionic compound and an organic polymer compound, wherein the organic polymer compound comprises a compound represented by the general formula (1) or a boric acid ester compound obtained by the esterification of the compound represented by the general formula (1) with boric acid or boric anhydride:
Z1xe2x80x94[(A1O)1xe2x80x94R1]axe2x80x83xe2x80x83(1)
wherein Z1 represents a residue of compound having from 1 to 6 hydroxyl groups; A1O represents one or a mixture of two or more of C2-C4 oxyalkylene groups; R1 represents a group selected from the group consisting of cyanoethyl group, C1-C12 hydrocarbon group and hydrogen atom; l represents an integer of from 0 to 600; and the suffix a represents an integer of from 1 to 6, with the proviso that la ranges from 0 to 600;
(b) The electrolyte for secondary battery according to Clause (a), wherein the organic polymer compound comprises a boric acid ester compound obtained by the esterification of a compound represented by the general formula (1) wherein R1 is a hydrogen atom with boric acid or boric anhydride;
(c) The electrolyte for secondary battery according to Clause (a), wherein the organic polymer compound comprises a nitrile group-containing compound represented by the general formula (1) wherein at least one of R1""s is a cyanoethyl group;
(d) An electrolyte for secondary battery comprising an ionic compound and an organic polymer compound, wherein the organic polymer compound comprises a polymerization product of a compound represented by the general formula (2) or a polymerization product of a boric acid ester compound obtained by the esterification of the compound represented by the general formula (2) with boric acid or boric anhydride:
Z2xe2x80x94[(A2O)mxe2x80x94R2]bxe2x80x83xe2x80x83(2)
wherein Z2 represents a residue of compound having from 1 to 4 hydroxyl groups; A2O represents one or a mixture of two or more of C2-C4 oxyalkylene groups; m represents an integer of from 0 to 150; b represents an integer of from 1 to 4, with the proviso that mb ranges from 0 to 300; and R2 represents a hydrogen atom, cyanoethyl group or a group represented by the general formula (3): 
wherein R3 and R4 each represent a hydrogen atom or methyl group;
(e) The electrolyte for secondary battery according to Clause (d), wherein the organic polymer compound comprises a polymerization product of a boric acid ester compound obtained by the esterification of a compound represented by the general formula (2) wherein R2 is a hydrogen atom with boric acid or boric anhydride;
(f) The electrolyte for secondary battery according to Clause (d, wherein the organic polymer compound comprises a nitrile group-containing compound represented by the general formula (2) wherein at least one of R2""s is a cyanoethyl group;
(g) The electrolyte for secondary battery according to Clause (a), wherein the organic polymer compound further comprises a polymerization product of a compound represented by the general formula (4):
Z3xe2x80x94[(A3O)nxe2x80x94R5]cxe2x80x83xe2x80x83(4)
wherein Z3 represents a residue of compound having from 1 to 4 hydroxyl groups or a hydroxyl group; A3O represents one or a mixture of two or more of C2-C4 oxyalkylene groups; n represents an integer of from 0 to 150; c represents an integer of from 1 to 4, with the proviso that nc ranges from 0 to 300; and R5 represents a hydrogen atom or a group represented by the general formula (5): 
wherein R6 and R7 each represent a hydrogen atom or methyl group;
(h) The electrolyte for secondary battery according to Clause (a) or (g), wherein the organic polymer compound further comprises an organic polymer compound represented by the general formula (6):
Z4xe2x80x94[(A4O)0xe2x80x94H]dxe2x80x83xe2x80x83(6)
wherein Z4 represents a residue of compound having from 1 to 6 hydroxyl groups; A4O represents one or a mixture of two or more of C2-C4 oxyalkylene groups; o represents an integer of from 100 to 150,000; and d represents an integer of from 1 to 6, with the proviso that od ranges from 100 to 150,000;
(i) The electrolyte for secondary battery according to Clause (a), wherein the organic polymer compound comprises a polymerization product of a nitrile group-containing compound represented by the general formula (2) or a polymerization product of a boric acid ester compound obtained by the esterification of a nitrile group-containing compound represented by the general formula (2) with boric acid or boric anhydride;
(j) The electrolyte for secondary battery according to Clause (d), wherein the organic polymer compound further comprises a polymerization product of a compound represented by the general formula (4);
(k) The electrolyte for secondary battery according to Clause (i), wherein the organic polymer compound further comprises a polymerization product of a compound represented by the general formula (4);
(l) The electrolyte for secondary battery according to Clause (d), (j) or (k), wherein the organic polymer compound further comprises an organic polymer compound represented by the general formula (6);
(m) The electrolyte for secondary battery according to Clause (a), (d), (g), (h), (i), (j), (k) or (l), wherein the ionic compound is an alkaline metal salt or alkaline earth metal salt;
(n) The electrolyte for secondary battery according to Clause (m), wherein the ionic compound is a lithium salt; and
(o) A secondary battery comprising an electrolyte for secondary battery defined in Clause (a), (d), (g), (h), (i), (j), (k), (l), (m) or (n).